3d imaging system and method for signaling an object of interest in a volume of data

ABSTRACT

The invention concerns a medical imaging system comprising means ( 2 ) of acquiring at least one volume of 3D data ( 3 DV), means ( 3 ) of detecting at least one object of interest in said volume of data, display means ( 4 ) able to supply a 2D representation ( 2 DR) of said volume of data and signaling means ( 5 ) intended to signal a location of said object of interest by means of a signal (SIG) superimposed on said 2D representation.

FIELD OF THE INVENTION

The present invention relates to a medical imaging system intended toform a 2D representation of an object of interest from an acquisition ofa volume of 3D data. It also relates to a method implemented by such asystem. Finally, it relates to a computer program product implementingsuch a method.

It finds an application especially in the medical field, in particularfor ultrasonic imaging and magnetic resonance imaging.

BACKGROUND OF THE INVENTION

3D imaging systems have developed a great deal during the past fewyears, including in the medical field. Consequently a doctor is more andmore induced to make a diagnosis, for example to seek an object ofinterest, from a volume of 3D data, a 2D representation of which heviews on a screen. Such a volume comprises more information than asimple 2D image and makes it possible to detect objects of interest thatcan scarcely be discerned on a 2D image. On the other hand, it is alsomore difficult to manipulate. This is because, unlike an image, not allthe data are simultaneously available on a single 2D representation ofthe volume. The doctor is to navigate in the volume and display severaldifferent 2D representations of this volume. He therefore needs anincreased amount of time to scan the volume exhaustively and make hisdiagnosis.

SUMMARY OF THE INVENTION

It is an aim of the present invention to propose a solution for makingthe visual detection by a user of an object of interest within a volumeof 3D data more reliable and more rapid, in particular in the medicalfield.

This aim is achieved by a medical imaging system comprising:

-   -   acquisition means intended to acquire at least one volume of 3D        data,    -   means of detecting at least one object of interest in said        volume of data, intended to supply characteristics of said        object,    -   means of displaying said volume of data intended to provide a 2D        representation of said volume,    -   signaling means intended to signal a location of said object of        interest from said characteristics, using a signal superimposed        on said 2D representation.

The system according to the invention signals to the user, by means of asound or a color, that he is displaying a 2D representation comprising apossible location of the object of interest. Such signals attract hisattention to this possible location of the object of interest. The usercan possibly move accordingly in the volume of 3D data in order todisplay the object of interest at another angle. This signaling isparticularly advantageous in the case where, as in the medical field,the object of interest is often difficult to detect to the naked eye andmay not be detected by a doctor. Alerted to all the locations where oneor more objects of interest may be situated, the doctor can concentratehis energy on the observation of a 2D representation rather than onnavigation in the 3D volume. The system according to the inventiontherefore has the advantage of guiding the user when he is navigating inthe volume of 3D data. Such a system also has the advantage of sparingthe user from having to navigate in the volume exhaustively.

Another advantage of the system according to the invention is to injectinto a 2D representation of a volume of 3D data characteristics relatedto the object of interest which cannot be obtained from the 2Drepresentation alone but require on the contrary apprehending the volumeof 3D data as a whole. This is the case, for example, when the region ofinterest is liable to comprise objects which are both spherical andtubular in shape. A 2D representation of the 3D volume may in this caseexhibit a more or less circular cross-section of the object, making itdifficult to distinguish between a spherical object and a tubularobject. The detection means according to the invention are able tosupply a characteristic of the object of interest such as itsorientation. Such a characteristic enables the user to recognize atubular object having a favored orientation from a spherical object nothaving any particular orientation.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described with reference to examples ofembodiments shown in the drawings to which, however, the invention isnot restricted.

FIG. 1 presents a functional diagram of an ultrasonic imaging systemaccording to the invention,

FIG. 2 illustrates the effect of a subtractive median filter used by thedetection means of the system according to the invention, in the case ofa 1D profile,

FIG. 3 illustrates the principle used by the derivation sub-meansaccording to the invention, in the case of a non-noisy 1D profile,

FIG. 4 illustrates the principle used by the derivation sub-meansaccording to the invention, in the case of a noisy profile,

FIG. 5 a presents an example of a tubular object of interest and theorientation of the particular vectors of the structure tensor supplyingthe principal axes of the object,

FIG. 5 b presents a possible choice of a display axis and of threeorthogonal views for constructing a 2D representation of a volume of 3Ddata according to the invention,

FIG. 6 presents an example of a 2D representation of a volume of 3D dataaccording to the invention,

FIG. 7 presents an example of microcalcification signaled in a 2Drepresentation of a volume of 3D data according to the invention,

FIG. 8 presents an example of a tubular structure signaled in a 2Drepresentation of a volume of 3D data according to the invention,

FIG. 9 presents a functional diagram of a magnetic resonance imagingsystem according to the invention,

FIG. 10 presents three contrast change curves in a delimited zone of aregion of interest over time.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 depicts a functional diagram of a 3D imaging system according tothe invention, in the medical field. In a first embodiment, anultrasonic imaging system for the detection of microcalcifications ofthe breast is considered. Such a system comprises means 2 of acquiring avolume 3DV of ultrasonic data 3D of a region of interest 1 of the humanbody, for example a breast, means 3 of detecting objects of interest,for example microcalcifications MC, in said volume 3DV, display means 4intended to deliver a 2D representation 2DR of the volume 3DV and means5 of signaling the microcalcifications MC in the representation 2DR.

The acquisition means 2 are able to emit ultrasonic signals 8 in thedirection of the region of interest 1 by means of a probe 7 and toreceive delayed ultrasonic signals 9 in return, the said delayed signalsbeing returned by the region of interest 1. The probe 7 compriseselements which are capable of converting an electrical pulse into asound wave and to receive a response returned by the region of interest.The said elements can be assembled in a matrix in order to form atwo-dimensional probe or in an array to form a one-dimensional probe. Ifthe probe is a matrix of elements, a 3D volume of ultrasonic data isacquired directly. In the case of a 1D probe, that is to say an array ofelements, conventional echographic imaging, according to a method knownto persons skilled in the art, provides, for a given position of theprobe, an image representing a 2D section of the environment in theplane of the probe. By then moving the probe, several sections throughthe same environment are obtained. All these sections constitute a 3Dvolume of data.

The volume 3DV obtained supplies a cartography of the ultrasonic energyreturned by the environment formed by the region of interest. The regionof interest is liable to comprise zones which return more or lessenergy. It is said that these zones are more or less echogenic. Someobjects of interest, such as microcalcifications MC, are point-sourceobjects, very echogenic, which appear as small bright points in thevolume 3DV. One difficulty in locating these microcalcifications in thevolume 3DV is that they are generally masked by a noise called“speckle”, which makes them difficult to detect with the naked eye.

The system according to the invention comprises detection means 3intended to detect objects of interest in the volume 3DV of ultrasonicdata. In one embodiment of the invention, the said detection means 3comprise median filtering sub-means, which consist of applying asubtractive median filter to the volume of data 3DV in order to enhanceobjects of interest of small size such as microcalcifications. FIG. 3illustrates the principle of subtractive median filtering in the 1Dcase. A profile y(x) of a microcalcification MC is depicted therein. Themicrocalcification MC forms a narrow peak surrounded by peaks of lesserintensity due to noise.

Effecting a median filtering at a point y(x₀) of the profile on afiltering window FF of width I consists of:

-   -   sorting the I values of the profile (y(x₀−½), . . . , y(x₀), . .        . , y(x₀+½−1)),    -   extracting the median value y_(m),    -   taking the median value y_(m) for the filtered value y′(x₀),    -   renewing the operation at each point on the profile.

The effect of such a filtering is to make the peak due to themicrocalcification MC disappear, provided that the filtering window FFis sufficiently wide compared with the width of the peak. In a secondstep, the median profile y′ is subtracted from the original profile y,which has the effect of dispensing with the low-frequency variations inthe profile whilst preserving the contrast at the microcalcification.The profile y-y′ reveals an enhanced microcalcification MCR.

In the case of a volume of data such as the volume 3DV, a median filter3D is used. In this case, the filtering window FF is a rectangularparallelepiped, for example a cube. Its size is chosen according to atemplate of objects of interest sought. Because of the non-idealresponse of the imaging system, a point-source object is represented bya spot which is not necessarily isotropic, that is to say which may bedeformed in some directions rather than in others. To take account ofthis defect in focusing, it may be necessary to consider a non-cubicparallelepipedal filtering window.

A volume of filtered data is obtained in which the structurescorresponding to the template are enhanced. The detection meansaccording to the invention comprise thresholding sub-means intended toextract the structures with the highest contrast from amongst theenhanced structures. The threshold is in particular chosen according tothe power of the noise present in the ultrasonic data. Afterthresholding, a location of the structures retained is easily derived.As a characteristic CAR of an object of interest detected, the detectionmeans according to the invention supply for example a position (x_(oi),y_(oi), z_(oi)) of the object of interest in a reference frame (O,x,y,z)of the volume 3DR.

In a second embodiment of the invention, objects of interest of elongateshape, for example tubular, are sought. In the field of medical imaging,it is a case for example of blood vessels, milk ducts, ligaments etc. Inthe case of echography of the breast, it is advantageous to be able tolocate the objects of interest of elongate shape in order to excludethem from potential microcalcifications and to know their orientation.In order to detect such anisotropic structures, the detection means 3according to the invention comprise sub-means of deriving the volume ofdata 3DV. The principles used by the said derivation sub-means areillustrated by FIG. 3 in the case of a non-noisy 1D profile Pr and byFIG. 4 in the case where the profile Pr is noisy. A Gaussian convolutionkernel g₀ is first of all applied to the profile Pr so as to filter thenoise. According to a technique known to persons skilled in the art,said derivation means then consist of calculating a second derivative,in order to reveal an object of interest having a contrast peak in theprofile Pr. This is because, since a first derivative is canceled out atthe location of the crests sought, a second derivative is preferred,since it has a maximum absolute value at the location of the saidcrests. This second derivative is then squared and then post-filtered bya Gaussian kernel g₁. It is used to detect the presence of a crest, thatis to say a one-dimensional contrast peak. An example of a peak P and asquare wave Cr is presented in FIGS. 3 and 4. It is clear that thesecond derivative enhances the peak P and to a lesser extent detects theedges of the square wave Cr whilst considerably reducing the power ofthe noise.

In three dimensions the detection means 3 make it necessary to calculateall the second derivatives along the three axes x,y,z of the referenceframe (O, x, y, z), which makes it possible to derive the Hessian matrixassociated with all the points of the volume 3DV: $H = \begin{bmatrix}{{\frac{\delta^{2}}{\delta\quad x^{2}} \otimes g_{0}}{\frac{\delta^{2}}{\delta\quad x\quad\delta\quad y} \otimes g_{0}}{\frac{\delta^{2}}{\delta\quad x\quad\delta\quad z} \otimes g_{0}}} \\{{\frac{\delta^{2}}{\delta\quad y\quad\delta\quad x} \otimes g_{0}}{\frac{\delta^{2}}{\delta\quad y^{2}} \otimes g_{0}}{\frac{\delta^{2}}{\delta\quad y\quad\delta\quad z} \otimes g_{0}}} \\{{\frac{\delta^{2}}{\delta\quad z\quad\delta\quad x} \otimes g_{0}}{\frac{\delta^{2}}{\delta\quad z\quad\delta\quad y} \otimes g_{0}}{\frac{\delta^{2}}{\delta\quad z^{2}} \otimes g_{0}}}\end{bmatrix}$

A tensor of structure T=(H.H^(T)){circle around (×)}g₁ is nextcalculated. A thresholding of the trace of the tensor T makes itpossible to retain the structures with the highest contrastcorresponding amongst other things to the tubular structures sought. Thethreshold is chosen according to a statistic of the noise liable tointerfere with the trace of the tensor.

The tensor T being a positive defined matrix, it has three real positiveproper values λ₁, λ₂ and λ₃, with λ₁<λ₂<λ₃, associated with three propervectors

₁,

₂ and

₃ forming a proper base aligned on the object of interest. An example ofa tubular structure is presented in FIG. 5 a. The proper vector

₁ associated with the smallest proper value λ₁ indicates the directionof the object of interest in the case of a tubular object. The saidderivation sub-means also make it possible to assess whether the objectof interest is isotropic or anisotropic from ratios between propervalues:

-   -   if λ₁≈λ₂≈₃ and λ₁ is large, the object of interest is highly        contrasted and has no favored direction. This is known as a        blob,    -   if λ₃/λ₁ is large, the object of interest has a favored        direction,    -   if λ₁≈λ₂ and λ₃ is large, the object of interest is a plane.

The object of interest can then be characterized not only by a location(x_(oi), y_(oi), z_(oi)) but also by an orientation. This orientation isfor example given by the proper vector

₁. It is also possible to calculate a measurement of angle α between

₁ and a vector normal to a section through the volume 3DV.

The display means 4 of the imaging system according to the inventionform a 2D representation 2DR of the volume of 3D data. In a preferredembodiment of the invention, the 2D representation 2DR comprises 3orthogonal sections or views Vw₁, Vw₂ and Vw₃. These three views aredefined along a display axis z′ in the following manner:

-   -   the view Vw₁ is orthogonal to the axis z′ and cuts the volume at        a depth z₀′,    -   the views Vw₂ and Vw₃ are orthogonal to each other and to the        view Vw₁ and pass through the axis z′.

FIG. 5 b illustrates a possible choice of the display axis z′ and of thethree orthogonal views Vw₁, Vw₂ and Vw₃. An example of a 2Drepresentation 2DR is presented in FIG. 6. It should be noted that thedisplay axis z′ is not necessarily parallel to the axis z of thereference frame (O, x, y, z).

In the preferred embodiment, the imaging system according to theinvention comprises check means 6 for checking a position of the saiddisplay axis in the said volume 3DV and a position of said first viewVw₁ along said axis z′. The positions of the other two views Vw₂ and Vw₃are modified accordingly. The user can therefore navigate in the volumeby choosing a position of the display axis z′ and a position of the viewVw₁ on this axis. When he quickly varies the coordinate z′ of the viewVw₁ he obtains a sequence called a “cineloop”.

The signaling means 5 of the imaging system according to the inventionare intended to signal a location of said object of interest in said 2Drepresentation, by means of a signal SIG superimposed on therepresentation 2DR. It is a case of alerting a user to the presence ofan object of interest in the volume 3DV and more precisely indicating tohim that the object of interest is visible on the representation 2DRwhich it is in the process of displaying. To do this, the signalingmeans use the characteristics CAR supplied by the detection means 3.

In the case of an isotropic object of interest, for example amicrocalcification, the characteristics CAR supplied by said detectionmeans may be a location defined by coordinates in the reference frame(O, x, y, z). The signaling means 5 then consist of superimposing thesignal SIG on the representation 2DR when said location is included inone of the three views Vw₁, Vw₂ or Vw₃ contained in the representation2DR. This signal SIG may be visual and appear on the view concerned as acolored shape, for example a circle centered on said location, as shownby FIG. 7 for a microcalcification MC. It may equally well be audible,that is to say a bleep is emitted when the user defines, using the checkmeans, a representation 2DR where one of the views cuts the object ofinterest.

It should be noted that any other signal SIG able to alert the user maybe used, for example a flash.

In the case of an anisotropic object having an orientation, for examplea blood vessel, the signal SIG may be an arrow representing theorientation of said vector or a color coding the measurement of angle αsuperimposed on a section 2DR of the volume 3DR, as shown by FIG. 8 fora tubular structure ST.

In a third embodiment of the invention, a magnetic resonance imagingsystem presented in FIG. 9 is considered. Magnetic resonance imaginguses a variable magnetic field. By a principle known to persons skilledin the art, the response of the environment studied to this excitationis recorded by the system and a sequence of sections of the region ofinterest is acquired, so as to form a volume of 3D data.

Such a system makes it possible to display soft tissues. It is inparticular used for imaging the breast and detecting any mammarylesions. For this purpose, the acquisition means 12 are able to effect adynamic acquisition of n volumes of data 3DV′(t), t=t₀, t₁ . . . t_(n−1)at n discrete times.

The dynamic acquisition aims to follow the diffusion of a contrastproduct, generally gadolinium, within the region of interest. Thisproduct, injected at time t₀, has the property of creating a contrastflash in a highly perfused zone of the region of interest, for example amammary lesion. It is said that the lesion “adopts the contrast”.However a lesion adopts the contrast differently depending on whether itis a case of a benign or malignant lesion. In other words, the speed atwhich the contrast product invades and leaves the lesion is not the samewhatever the type of lesion encountered. It is therefore advantageous tolook at the propagation of the contrast product at successive times t₀,t₁, t₂ . . . t_(n−1) and to assess its dynamics over time. Between timesto and ti, adoption of contrast is referred to. The contrast productprogressively invades the region of interest and emphasizes any lesions.FIG. 10 depicts examples of curves of change in contrast Ct in theregion of interest. Between times t₁, and t₂, three main scenarios arepossible:

-   -   either the quantity of contrast product present at the point of        the lesion continues to increase, as indicated by the curve 20.        A phenomenon of wash-in (W_(in)) is referred to and, in this        case, a benign lesion is often involved,    -   or the quantity of contrast product stagnates, as indicated by        the curve 21,    -   or the quantity of contrast product falls, as indicated by the        curve 22. A phenomenon of wash-out (W_(out)) is referred to and,        in this case, probably a malignant lesion is involved.

The display means 13 of the system according to the invention enable thedoctor to display one or more volumes of data 3DV(t) obtained atdifferent times t in the form of sections through this volume. The checkmeans 14 enable him to choose a section Vw₁′(t) where he has isolated anobject of interest, for example a lesion, and thus to display the changein contrast on this section.

The purpose of the detection means 15 is to reveal any phenomena ofwash-in and wash-out. Said means comprise local mean calculation means.It may be a case either of a spatial mean on the chosen 2D sectionsVw₁′(t) at all points on the said sections, or a spatial mean on thevolumes 3DV(t) at any point on said volumes. Said means also comprisesub-means of calculating the contrast slope between two successive timest_(i) and t_(i+1), at all points on said sections or said volumes.

The sub-means of calculating the mean consist of evaluating a localmean. Consider a point on a volume 3DV′(t), the local mean at this pointis obtained by summing, in a vicinity V centered on the point processed,all the values of points of the volume belonging to V sufficiently closeto the value of the point. This requires extracting in the vicinity V arelated sub-vicinity SV whose values meet homogeneity criteria andaveraging the values contained in SV. There exist a great variety ofapproaches for extracting homogeneous zones in a vicinity. Suchapproaches involve segmentation techniques known to persons skilled inthe art. The sub-means for calculating the slope effect the subtraction,between two consecutive times, of the values of local means describedabove and supply a measurement of the contrast slope, that is to say anevaluation of the speed of propagation of the contrast product in thearea of interest between times t₁ and t₂. A positive slope between thetimes t₁ and t₂ indicates a wash-in phenomenon whilst a negative slopeindicates a wash-out phenomenon.

The doctor generally displays a sequence Vw₁′(t) or a particular view ofthe sequence and the curve representing the contrast slope in parallel.The signaling means 16 of the system according to the invention make itpossible to display directly an adoption or loss of contrast at anypoint on the section observed either by superimposing, or by displayingseparately, a coloring whose code corresponds to the speed ofpropagation between two consecutive times. This makes it possible inparticular to convert the wash-in and wash-out indices into signalswhich are superimposed on the section through the volume 3DV observed.For example, it is possible to color in red in the case of wash-in andblue in the case of wash-out. One advantage of the signaling means 16according to the invention is to add time information to a 2Drepresentation of an anatomical acquisition of a volume of data3DV′(t₀). All the information available to it are calculated at everypoint in the volume and can therefore be grouped together on the samepage for a given section through the volume 3DV, in order to facilitatethe making of a diagnosis.

In a fourth embodiment, the system according to the invention comprisesmeans of storing the volume of 3D data able to store said volume in theform of a collection of representations 2DR.

In the case of an ultrasonic imaging system as described in the firstand second embodiments, the system according to the invention makes itpossible in fact to define one or more representations 2DR revealing theobject of interest. These representations 2DR have been defined by thedoctor using the check means 6 and signaling means 5. It can beconsidered that these representations group together the data of thevolume 3DV which are truly useful to the doctor in order to make adiagnosis and that said representations can advantageously be stored inplace of the volume of 3DV or in addition to it.

In the case of a magnetic resonance imaging system as described in thethird embodiment of the invention, it can be considered that thesequence Vw₁′(t) or even a particular image in this sequence, combinedwith the wash-in and wash-out indices, group together all the datauseful to the doctor for making his diagnosis and can thereforeadvantageously supplement the n volumes of data 3DV′(t) or even replacethem.

One advantage of the system according to the invention is therefore toafford savings in storage of the volumes of data 3DV or 3DV′(t)acquired.

The major advantage of said storage means is to facilitate any newaccess to the data. This is because, when a doctor wishes to consult amedical file comprising data obtained by means of a 3D imaging system,he is not obliged to waste time navigating in the volume 3DV. Therepresentations 2DR which were stored concentrate all the useful data.

The invention is not limited to the embodiments which have just beendescribed by way of example. Modifications or improvements can be madethereto whilst remaining within the scope of the invention. Inparticular, other imaging modes, such as X-ray imaging, can be used.

In the claims the verb “comprise” is used to signify that the use ofother elements, means or steps is not excluded.

1. A medical imaging system comprising: acquisition means intended toacquire at least one volume of 3D data, means of detecting at least oneobject of interest in said volume of data, intended to supplycharacteristics of said object, means of displaying said volume of dataintended to provide a 2D representation of said volume including atleast a portion of the detected object of interest, signaling meansintended to signal a location of said object of interest from saidcharacteristics, using a signal superimposed on said 2D representation.2. A system as claimed in claim 1, characterized in that said 2Drepresentation comprises a first section through said volume of data,said first section being orthogonal to a display axis, a second sectioncomprising said axis and orthogonal to the first section and a thirdsection comprising said axis and orthogonal to the first and secondsections, and in that said location of the object of interest is a zoneof intersection of the object of interest with said first, second orthird section.
 3. A system as claimed in claim 2, characterized in thatit comprises check means for checking a position of said display axis insaid volume and a position of said first section along said axis.
 4. Asystem as claimed in claim 2, characterized in that said signaling meansare able to emit a sound in order to signal the presence of said objectof interest in said 2D representation.
 5. A system as claimed in claim2, characterized in that said signaling means are able to mark saidintersection zone by a color.
 6. A system as claimed in claim 1,characterized in that said system is an ultrasonic imaging system.
 7. Asystem as claimed in claim 6, characterized in that, the object ofinterest being a tubular object comprising an orientation, saiddetection means are able to supply a measurement of said orientation andsaid signaling means are able to signal the location of said object ofinterest on said 2D representation and its orientation.
 8. A system asclaimed in claim 1, characterized in that, said system being a magneticresonance imaging system intended to follow a propagation of a contrastproduct in said region, said detection means are able to calculate aspeed of propagation of said product through said object of interest andsaid signaling means are able to signal the location of said object onsaid 2D representation by superimposing on it a signal indicating saidspeed of propagation.
 9. A system as claimed in claim 1, characterizedin that it also comprises means of storing the volume of 3D data able tostore said volume in the form of a collection of 2D representations,said 2D representations comprising said signal.
 10. A method fordisplaying at least one object of interest in a medical diagnosticimage, comprising the steps of: acquiring at least one volume of 3Ddata, detecting said object of interest in said volume of data in orderto supply characteristics of said object, displaying said volume of datain order to supply a 2D representation of said volume which includes atleast a portion of said detected object of interest, signaling forsignaling a location of said object of interest in said 2Drepresentation using said characteristics.
 11. A computer programproduct for implementing a method as claimed in claim
 10. 12. A systemas claimed in claim 1, wherein the object of interest has a boundarywithin the volume of data; and wherein the signaling means signals alocation of said object of interest using a signal superimposed on said2D representation within the boundary of the object of interest.
 13. Asystem as claimed in claim 12, wherein the signal superimposed on said2D representation produces a distinctive color within the boundary ofthe object of interest in comparison to surrounding anatomy.
 14. Asystem as claimed in claim 12, wherein the signal superimposed on said2D representation produces a distinctive contrast within the boundary ofthe object of interest in comparison to surrounding anatomy.